High solubility iron hexacyanides

ABSTRACT

Stable solutions comprising high concentrations of charged coordination complexes, including iron hexacyanides are described, as are methods of preparing and using same in chemical energy storage systems, including flow battery systems. The use of these compositions allows energy storage densities at levels unavailable by other iron hexacyanide systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/166,174, filed on May 26, 2016, which is a divisional of U.S. patentapplication Ser. No. 13/887,461, filed on May 6, 2013, which is acontinuation-in-part of International Patent ApplicationPCT/US2013/030430, filed on Mar. 13, 2013, which claims priority to U.S.Provisional Patent Application 61/683,260, filed on Aug. 15, 2012, eachof which is incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure is in the field of energy storage systems, includingelectrochemical cells and flow battery systems, and methods of operatingthe same. In particular, the invention relates to solutions havingferrocyanide/ferricyanide concentrations higher than previously observedor employed, and the use thereof in energy storage systems.

BACKGROUND

The ferrocyanide/ferricyanide redox couple, Fe(CN)₆ ^(3−/4−), is wellunderstood and is frequently used in energy storage applications, butthe low solubilities of available salts has limited its use, owing tothe low associated energy densities. For example, the solubilities ofNa₄Fe(CN)₆.10H₂O, K₄Fe(CN)₆.3H₂O, and Ca₂Fe(CN)₆.11H₂O in water atambient temperatures are listed in Ullmann's Encyclopedia of IndustrialChemistry as 33.7 g, 33.7 g, and 148.4 g in 100 g of water, respectively(other sources list similar or lower values for these solubilities).These correspond to concentrations of about 0.7 M, 0.8 M, and 3 M,respectively. Given these limits, energy storage systems use the Fe(CN)₆^(3−/4−) couple at concentrations lower than these at ambienttemperature at all pH ranges (and are typically not greater than 0.52M). While the use of alkaline earth metal salts may provide higherconcentrations at neutral pH, their use in alkaline systems isdisfavored by the precipitation of metal hydroxides—e.g., Ca(OH)₂.

Prior efforts to use this ferrocyanide couple in energy storage systemsgenerally seek to overcome the inherent solubility limits of Na₄Fe(CN)₆or K₄Fe(CN)₆ systems by engineering means and/or by operating systems atelevated temperatures. For example, one group explored the use ofelaborate flow-through crystallizers in the electrolyte stream toincrease the energy density of the solution from the 0.5-0.6 M [Fe(CN)₆]dissolved in the liquid phase by separating out insoluble crystallites.See Hollandsworth, R. P., et al., “Zinc/Ferrocyanide Battery DevelopmentPhase IV” Lockheed Missiles and Space Company, Inc., contractor report,Sandia Contract DE-AC04-76DP00789, 1985.

SUMMARY

The present inventions are directed to solutions of iron hexacyanidesand their use, for example, in chemical energy storage systems,including flow battery systems. These solutions allow for energy storagedensities at levels unavailable by other systems.

Various embodiments of the present invention provide solutions,preferably stable solutions, each of which comprises: (a) a chargedmetal-ligand coordination complex; and (b) at least two differentcounterions; the concentration of said coordination complex, at a giventemperature, being higher than can be obtained when said coordinationcomplex is in the presence of any single one of the at least twodifferent counterions. In certain embodiments, the (stable) aqueoussolution comprises iron(II) hexacyanide, [Fe(CN)₆ ⁴⁻], in the presenceof sodium and potassium ions, whereby the concentration of Fe(CN)₆ ⁴⁻ insaid solution, at a given temperature, exceeds the concentration ofFe(CN)₆ ⁴⁻ in either a saturated solution of Na₄[Fe(CN)₆] or a saturatedsolution of K₄[Fe(CN)₆], at the same temperature. In other embodiments,the (stable) aqueous solution comprises iron(III) hexacyanide, [Fe(CN)₆³⁻], in the presence of sodium and potassium ions, whereby theconcentration of Fe(CN)₆ ³⁻ in said solution, at a given temperature,exceeds the concentration of Fe(CN)₆ ³⁻ in either a saturated solutionof Na₃[Fe(CN)₆] or a saturated solution of K₃[Fe(CN)₆], at the sametemperature.

Other embodiments provide methods of preparing an aqueous solution,preferably a stable aqueous solution of iron(II) hexacyanide, [Fe(CN)₆⁴⁻], each method comprising dissolving sufficient amounts ofNa₄[Fe(CN)₆] and K₄[Fe(CN)₆] in an amount of aqueous solvent (preferablywater substantially free of co-solvents), so as to provide aconcentration of Fe(CN)₆ ⁴⁻ in said solution, at a given temperature,that exceeds the concentration of Fe(CN)₆ ⁴⁻ in either a saturatedsolution of Na₄[Fe(CN)₆] or a saturated solution of K₄[Fe(CN)₆], at thesame temperature.

Still further embodiments provide methods of preparing, and theresulting aqueous solutions, preferably stable aqueous solutions ofiron(II) hexacyanide, [Fe(CN)₆ ⁴⁻], each method comprising mixingsufficient amounts of H₄[Fe(CN)₆], NaOH, and KOH in sufficient water, soas to provide a concentration of Fe(CN)₆ ⁴⁻ in said solution, at a giventemperature, that exceeds the concentration of Fe(CN)₆ ⁴⁻ in either asaturated solution of Na₄[Fe(CN)₆] or a saturated solution ofK₄[Fe(CN)₆], at the same temperature.

Additional embodiments provide methods of preparing, and the resultingaqueous solutions, preferably stable aqueous solutions of iron(II)hexacyanide, [Fe(CN)₆ ⁴⁻], each method comprising: (a) mixing sufficientamounts of Ca₂[Fe(CN)₆], NaOH, and KOH in an amount of water, so as toprovide a concentration of Fe(CN)₆ ⁴⁻ in said solution, at a giventemperature, that exceeds the concentration of Fe(CN)₆ ⁴⁻ in either asaturated solution of Na₄[Fe(CN)₆] or a saturated solution ofK₄[Fe(CN)₆], at the same temperature; and (b) removing precipitatedCa(OH)₂.

The solutions resulting from any of these methods of preparing are alsoprovided as independent embodiments.

Similarly, analogous preparations and aqueous solutions, preferablystable aqueous solutions of iron(III) hexacyanide, [Fe(CN)₆ ³⁻] are alsoprovided, as are solutions obtainable by the interconvertability ofFe(CN)₆ ⁴⁻ and Fe(CN)₆ ³⁻ by either chemical and/or electrochemicalmethods.

In various embodiments, the invention provides electrolytes comprisingany of the solutions described herein, together with a supportingelectrolyte and/or at least one additional redox active material.Specific embodiments provide electrolytes, each electrolyte comprisingiron(II) hexacyanide, iron(III) hexacyanide, or a mixture of iron(II)hexacyanide and iron(III) hexacyanide capable of exhibiting atheoretical charge/discharge density of at least about 20 A-h/L.

In still further embodiments, the invention provides electrochemicalcells, including flow battery cells, each cell having at least onehalf-cell comprising a solution described herein, as well as energystorage systems comprising a series array of at least one suchelectrochemical cell.

Also provided are methods of operating such cells or systems, eachmethod comprising passing a current through said solution so as toeffect a change in the oxidation state of the metal-ligand coordinationcomplex and/or the iron hexacyanide complex.

The principles described herein, as well as the compositions orsolutions derived from these principles and comprising a chargedmetal-ligand coordination complex, generally, and iron hexacyanides,specifically, may be used in a wide variety of applications, includingbut not limited to energy storage; energy conversion; metal extractionfrom ores or other matrices; electro- and electroless plating; providingsoluble sources of cyanide or nitric oxide; electrochemical sensortechnology; device calibration by electrochemical, spectroscopic, ormagnetic means; the production of safety paper and other inks,dyestuffs, or dye formulations; animal feed supplements;electrochromics; and anti-caking agents.

BRIEF DESCRIPTION OF THE DRAWINGS

The present application is further understood when read in conjunctionwith the appended drawings. For the purpose of illustrating the subjectmatter, there are shown in the drawings exemplary embodiments of thesubject matter; however, the presently disclosed subject matter is notlimited to the specific methods, devices, and systems disclosed. Inaddition, the drawings are not necessarily drawn to scale. In thedrawings:

FIG. 1 illustrates a reference UV/visible spectrum of [Fe(CN)₆]⁴⁻,plotted a the molar extinction coefficient.

FIG. 2A and FIG. 2B are UV/visible spectra of diluted samples of 1.5 M[Fe(CN)₆]⁴⁻ as described in Example 1.

FIG. 3 shows the cyclic voltammogram of 1.5 M [Fe(CN)₆]⁴⁻ obtained at aglassy carbon disk working electrode at several scan rates using 0.1 Msodium potassium hydrogen phosphate as the supporting electrolyte, asdescribed in Example 2. The ratio of Na^(+/)K⁺ counterions in thisexample is ca. 1:1.

FIG. 4 shows the cyclic voltammogram of 1.0 M [Fe(CN)₆]⁴⁻ obtained at aglassy carbon disk working electrode at several scan rates with 2 Mhydroxide supporting electrolyte. The ratio of Na^(+/)K⁺ counterions inthis example is ca. 1:1.

FIG. 5 shows the increased charge/discharge capacity enabled in anenergy storage system using concentrated solutions of [Fe(CN)₆]⁴⁻ inaccordance with Example 3. The ratio of Na^(+/)K⁺ counterions in thisexample is ca. 1:1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing description taken in connection with the accompanying Figuresand Examples, all of which form a part of this disclosure. It is to beunderstood that this invention is not limited to the specific products,methods, conditions or parameters described and/or shown herein, andthat the terminology used herein is for the purpose of describingparticular embodiments by way of example only and is not intended to belimiting of any claimed invention. Similarly, unless specificallyotherwise stated, any description as to a possible mechanism or mode ofaction or reason for improvement is meant to be illustrative only, andthe invention herein is not to be constrained by the correctness orincorrectness of any such suggested mechanism or mode of action orreason for improvement. Throughout this text, it is recognized that thedescriptions refer to solutions, methods of making and using saidsolutions, devices and systems using said solutions, and methods ofoperating such devices and systems. That is, where the disclosuredescribes and/or claims a feature or embodiment associated with asolution, a composition comprising a solution, a method of making andusing a composition or solution, a device or system using a compositionor solution, or a method of operating such a device or system, it isappreciated that such a description and/or claim is intended to refer toall of these features or embodiment.

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art, and so forth.

When a value is expressed as an approximation by use of the descriptor“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function. The person skilledin the art will be able to interpret this as a matter of routine. Insome cases, the number of significant figures used for a particularvalue may be one non-limiting method of determining the extent of theword “about.” In other cases, the gradations used in a series of valuesmay be used to determine the intended range available to the term“about” for each value. Where present, all ranges are inclusive andcombinable. That is, references to values stated in ranges include everyvalue within that range.

When a list is presented, unless stated otherwise, it is to beunderstood that each individual element of that list and everycombination of that list is to be interpreted as a separate embodiment.For example, a list of embodiments presented as “A, B, or C” is to beinterpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A orC,” “B or C,” or “A, B, or C.”

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.That is, unless obviously incompatible or specifically excluded, eachindividual embodiment is deemed to be combinable with any otherembodiment(s) and such a combination is considered to be anotherembodiment. Conversely, various features of the invention that are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any sub-combination. Finally, while anembodiment may be described as part of a series of steps or part of amore general structure, each said step or part may also be considered anindependent embodiment in itself

The present invention includes embodiments related toferrocyanide/ferricyanide systems and use thereof. It also encompassescompositions/solutions and use embodiments of a more expansive nature.The inventors have discovered, for example, that certain mixed saltssystems provide solubilities significantly higher than those of theircorresponding single salt systems. For example, the present inventionallows for solutions of metal cyanide complexes that exhibit higherconcentrations of the cyanometallate ion than might be expected by oneof ordinary skill in the art given the published solubility limits forthe individual salts of interest.

Throughout this specification, words are to be afforded their normalmeaning, as would be understood by those skilled the relevant art.However, so as to avoid misunderstanding, the meanings of certain termswill be specifically defined or clarified.

For example, as used herein, the term “charged metal-ligand coordinationcomplex,” or simply “coordination complex,” refers to those complexescomprising a zero or non-zero valence transition metal (i.e., an elementhaving filled or unfilled d-orbitals, including members of groups 3 to12 in the periodic table, as well as members of the lanthanide andactinide series), having coordinated ligands, wherein the combination ofthe metal and ligands presents a non-zero charge, as would be understoodby the skilled artisan. Unless otherwise specified, the term“coordinated ligands” refers to any chemical moiety within thecoordination sphere of the metal. However, additional independentembodiments provide that these coordinated ligands are individuallyinorganic, organic, or mixed inorganic/organic, and are monodentate,bidendate, polydentate, or a combination thereof.

Also, unless otherwise specifically indicated, the term “counterion” isintended to connote those species whose formal charge sign is oppositeto that of the coordination complex, and so is capable of balancing thecharge of the metal-ligand coordination complex. Counterions includethose species which can then stabilize or effect the formation oflattice crystals of the metal-ligand coordination complex. The term“formal charge” is used to reflect that, under certain conditions, thecoordination complex and its associated counterions may exist insolution as ion pairs, rather than free ions, though this associationdoes not detract from the intended meanings.

Still further, as used herein, reference to a “stable solution” refersto a solution that is stable with respect to precipitation. Note thatthe optional parenthetical “(stable)” preceding the term “solution,” asused herein, is intended to connote individual embodiments comprisingthe solution and the solution which is stable with respect toprecipitation. As is known in the art, a dissolved species in a solutionthat is stable to precipitation does not sediment spontaneously, and anumber of tests may be relevant for determining the present or absenceof a precipitate. For example, such a species cannot be collected on a0.2 micron filter and typically forms symmetrical elution peaks whenpassed through a size exclusion chromatography column. A solution thatis not stable to precipitation may include a gross two phase system withsettled solids or a dispersion with a turbidity measurable against aFormazin Turbidity Unit (FTU) standard or an equivalent standard byknown light scattering methods (e.g., ISO 7027:1999). Such stabilitytesting may be done under any conditions deemed relevant for theintended use of the solution, but for present purposes, unless otherwisespecified, the term “stable solution” should be taken to mean that thesolution does not form a precipitate comprising the coordinationcomplex, as detectable by any of the preceding methods, when thesolution is left to stand at normal, ambient temperatures (e.g., in therange of from about 20° C. to about 25° C.) for about 30 days.

Additional individual embodiments also include those where the stabilityis defined at any given temperature in the range discussed below, for atime of at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, orabout 100 days or about one 1, 2, or about 5 years. For example, in anadditional embodiment, the solution is considered stable, if thesolution does not form a precipitate comprising the coordinationcomplex, as detectable either visually or by light scattering, when thesolution is left to stand at a refrigerated temperature (e.g., in therange of from about 0° C. to about 4° C.) for 7 days.

As used herein, then, the term “solution stability” is not intendednecessarily to refer to chemically stable solutions (i.e., resistant tochemical degradation), though this is a preferred characteristic of thestable solution.

Within these definitions, certain embodiments of the present inventionprovide for stable solutions, each of which comprises (or consistsessentially of): (a) a charged metal-ligand coordination complex; and(b) at least two different counterions; the concentration of saidcoordination complex, at a given temperature, being higher than can beobtained when said coordination complex is in the presence of any singleone of the at least two different counterions.

For example, for a system containing counterions A, B, and C, thesolubility of the coordination complex in the presence of A, B, and C,is greater than the solubility limit of the coordination complex in thepresence of only A, in the presence of only B, and in the presence ofonly C. For the sake of clarity, reference to “enhanced solubility” isintended to connote the condition where this condition is met. In thisregard, this enhanced solubility is an essential feature, the basic andnovel characteristic, of the invention. Therefore, where the embodimentsdescribed herein are described using the open-ended “comprising”language, such embodiments may be interpreted as also including thoseembodiments which may be described in terms of “consisting essentiallyof language,” with this enhance solubility as the basic and novelcharacteristic. Also, as understood by the skilled artisan, the term“solubility limit” refers to the amount of material (in this case, thecoordination complex and/or the ferro-/ferricyanide complex) that asolvent can hold at a given temperature before precipitation of thecomplex occurs; i.e., the point at which a certain material becomessaturated in the solvent, but not supersaturated.

It should be appreciated that, notwithstanding the use of the term “thatcan be obtained,” the comparison between the coordination complex in thepresence of “at least two different counterions” and in the presence of“any single one of the at least two different” is to be made betweensolutions having otherwise identical ingredients (e.g., additives) andunder otherwise identical circumstances (e.g., including temperature).

Further, specific embodiments include those where this effect may existwhether there are two or more different counterions, including thosesituations where the counterions may be nominally associated with thecoordination complex, but may also include ions from other materialsadded to the solution—e.g., associated with an added buffer orsupporting electrolyte. The term “supporting electrolyte” is definedbelow.

As used herein, the term “solution” carries its normal meaning, asunderstood by one skilled in the art—i.e., homogeneous mixture of asolid dissolved in a liquid. However, as used herein, the term“solution” is not intended to be read as necessarily requiring theabsence of other, non-dissolved materials, or a that the solution is thecontinuous phase of a mixture. That is, in the present context, a“(stable) aqueous solution of iron hexacyanide” would also be present ina mixture comprising particles suspended within a (stable) aqueoussolution of the iron hexacyanide and/or an emulsion or microemulsion inwhich the continuous or discontinuous phase comprises the (stable)aqueous solution.

Further, in addition to the coordination complex (including the ironhexacyanides) and the at least two counter-ions, a (stable) solution mayfurther comprise other ionizing or non-ionizing materials, which make itmore suitable for its intended application, but which do not interferewith the basic and novel characteristic of the invention. Ionizingmaterials (i.e., those with partially or completely ionize or form ionpairs in solution) may include, for example, supporting electrolytes(defined below), buffering agents, ionic (anionic, cationic, andzwitterionic) surfactants or detergents, and/or colligative property orpH adjusters. Exemplary ionizing materials include, but are not limitedto, strong or weak acids (including hydrochloric, nitric, phosphoric,sulfuric, or carboxylic acids, such as acetic, citric, amino acids, orEDTA) and bases (including hydroxides, amines, and the conjugate basesof the aforementioned acids); alkali metal, alkaline earth metal, orammonium salts; and salts of carboxylates (including acetic acid, citricacid, and EDTA), borates, halides (including bromide, chloride,fluoride, and iodide), nitrates, nitrites, sulfates, sulfites,phosphates, hydrogen phosphates, phosphites, polyphosphates. Exemplarybuffering agents include acetic acid, bicine, cacodylate buffer, CHES(2-(cyclohexylamino)-ethanesulfonic acid), citric acid, HEPES(4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid), MES(2-(N-morpholino)ethanesulfonic acid), MOPS(3-(N-morpholino)propanesulfonic acid), PIPES(piperazine-N,N′-bis(2-ethanesulfonic acid)), SSC (saline-sodium citratebuffer), TAPSO(3-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]-2-hydroxypropane-l-sulfonicacid), TRIS (2-amino-2-hydroxymethyl-propane-1,3-diol), and tricine

The solutions may also contain non-ionizing materials, for examplenon-ionic co-solvents (including water miscible or soluble alcohols,including C₁₋₃ alcohols, glycols, or polyglycols; ketones; oraldehydes), viscosity modifiers or gelling agents (including citrate,corn starch, corn syrup, gelatin, glycerol, guar gum, pectin), and/orwetting agents (including non-ionic surfactants and/or detergents).

Again, to the extent that these added materials effect solubility of thecoordination complex, it should be appreciated that any comparisonbetween the enhanced solubility of the coordination complex in thepresence of the at least two types of counterions and the singlecounter-ion should be made under the same compositional conditions.

Within the context of this invention, the (stable) solution ispreferably, though not necessarily, aqueous. Unless otherwise specified,the term “aqueous” refers to a solvent system comprising at least about98% by weight of water, relative to total weight of the solvent.However, in many applications, soluble, miscible, or partially miscible(emulsified with surfactants or otherwise) co-solvents may also beusefully present which, for example, extend the range of water'sliquidity (e.g., alcohols/glycols). When specified, additionalindependent embodiments include those where the “aqueous” solvent systemcomprises at least about 55%, at least about 60 wt %, at least about 70wt %, at least about 75 wt %, at least about 80%, at least about 85 wt%, at least about 90 wt %, at least about 95 wt %, or at least about 98wt % water, relative to the total solvent. It many situations, theaqueous solvent may consist of water, and be substantially free or freeof co-solvents.

While the invention includes embodiments where the (stable) solutionsare alternatively alkaline, acidic, or substantially neutral, in certainpreferred embodiments (e.g., including solutions offerro-/ferricyanide), the (stable) solutions of the coordinationcomplexes are alkaline. As used herein, unless otherwise specified, theterm “alkaline” refers to a solution having an apparent pH in excess ofabout 7. The term “apparent” is used to accommodate solvent systems thatare free of or contain a co-solvent, but (in the latter case) whichregister a pH in excess of about 7 when interrogated with a pH meter (pHmeter being exemplified by a device in which a voltmeter measures thepotential difference between a reference electrode and a sense electrodeheld in ionic contact with the solution of interest). While the term“alkaline” refers to a solution having an apparent pH in excess of 7,other embodiments of the invention include those where the pH orapparent pH is in the range of about 7 to about 14, and those where thepH or apparent pH is nominally greater than 14 (i.e., highly alkalinesystems—including multi-molar (e.g., 2 M) hydroxides). Additionalindependent embodiments also include those solutions in which the pH isat least about 7.5, 8, 8.5, 9, 9.5, 10, 11, 12, 13, or about 14 pH units(and upper end uncapped) and in which the pH is less than about 14,13.5, 13, 12.5, 12, 11.5, or 11 pH units; with exemplary ranges alsoincluding about 7-14, 8-13, 9-13, 10-13, 8-12, 8-11, and 9-12 pH units.

In certain embodiments, the non-zero charge associated with thecoordination complex is negative (i.e., the coordination complex itselfis anionic), in which case the relevant at least two counterions (givingrise to the enhanced solubility) are each positively charged (i.e., arecationic). These cationic counterions may comprise species having anynumber of formal positive charges, including mixtures of speciescarrying different positive charges, though mono- and di-cationic(including alkali and alkaline earth metal cations) species, or mixturesthereof, are preferred. In other preferred embodiments, the cationiccounterions are mixtures of mono-valent cations, for example, alkalimetal or optionally substituted (e.g., NH₄ ⁺ or mixed hydrogen, alkyl,aryl) ammonium cations. Combinations comprising Na⁺ and K⁺ appear to bemost preferred, at least for the ferro-/ferricyanide system.

In other embodiments, the non-zero charge associated with thecoordination complex is positive. Where the coordination complex carriesa positive charge, then, the associated counterions are anionic. In thiscase, various embodiments provide that the counterions may eachindependently carry any number of formal negative charges, includingsingly, doubly, or triply charged anionic species, or mixtures thereof.Such species may include, but are not limited to, chalcogenides, halides(including F⁻, Cl⁻, BC, F), hexa-alkyl or -aryl phosphates,hexafluorophosphate, nitrate, nitrite, perchlorate, phosphate, hydrogenphosphates, phosphonates, sulfate, sulfite, tetra-alkyl or -arylborates,tetrafluoroborates, or triflates.

In certain preferred embodiments, the (stable) solution comprises onlytwo types of counterions. Within these embodiments are included thoseembodiments wherein the molar ratio of the first and second of thesecounterions, independent of charge on each counterion, is in the rangeof from about 1:10 to about 10:1, with respect to one another. In otherindependent embodiments, this ratio is in the range of from about 1:8 toabout 8:1, about 1:6 to about 6:1, from about 1:4 to about 4:1, about1:3 to about 3:1, about 1:2 to about 2:1, about 1:1.5 to about 1.5:1, orsubstantially 1:1 (the term “substantially allowing for experimentalerror in measuring the salts or drift which may occur during the use ofthe solution). In those cases where the (stable) solution comprises morethan two types of counterions, additional embodiments provide that atleast one pair of the counterions satisfy the criteria set forth in thisparagraph.

The enhanced stability of the coordination complex in the presence of atleast two counterions is described above in terms of “a giventemperature;” i.e., “at a given temperature, the concentration of saidcoordination complex in the (stable) solution is higher than thesolubility of said coordination complex, when said coordination complexis in solution with any single one of the at least two differentcounterions.” This description is intended to connote both that thecomparison between the solubilities is being made at a single (and thesame) temperature, and that this condition of enhanced solubility issatisfied if present at any temperature. In many contexts, adetermination of this “given temperature” is made at normal ambient,room temperature (i.e., in the range of from about 20° C. to about 25°C.), simply for the sake of convenience in measuring the solubilities.However, in other applications such a determination may be made at anytemperature in the range of from about −20° C. to about 150° C. In otherembodiments, this temperature is one in the range bounded at the lowerend by a temperature of about −10° C., about 0° C., about 10° C., about20° C., about 30° C., or about 40° C., and at the upper end by atemperature of about 90° C. 80° C. 70° C., 60° C., 50° C., or about 40°C. Exemplary embodiments include, but are not limited to, those in therange of from about −10° C. to about 60° C., about 40° C. to about 80°C., or about 10° C. to about 30° C.

The (stable) solutions may also be described in terms of the enhancementof solubility which arises from the presence of the plurality (“at leasttwo types”) of counterions. In certain embodiments, then, the (stable)solution provides a concentration of the coordination complex which isat least about 10% higher than the solubility limit of said coordinationcomplex, when said coordination complex is in solution in the presenceof any and only one of the at least two types of counterions. In otherindependent embodiments, the concentration of the coordination complexis at least about 25% higher, at least about 50% higher, at least about75% higher, or at least about 100% higher than the solubility limit ofsaid coordination complex, when said coordination complex is in solutionin the presence of any and only one of the at least two types ofcounterions.

To this point, the coordination complex has been described only in termsof a charged complex comprising a transition metal having coordinatedligands. In certain embodiments, the coordination complex is redoxactive. That is, in separate embodiments, the coordination complex iscapable of undergoing oxidation, reduction, or both oxidation andreduction, with the application of electric current (or a voltage to asuitable electrode surface in contact with a solution comprising thecoordination complex) or an appropriate chemical oxidizing agent orreducing agent. In this context, the term “redox active” means that thecoordination complex exhibits an oxidation or reduction potential, inaqueous solution, in the range of from about −0.8 V to about 1.8 V vs.RHE.

The iron hexacyanide system provides specific embodiments within thisinvention. That is, certain additional embodiments include those whereinthe coordination complex is an iron hexacyanide and two of the at leasttwo types of counterions are alkali metal cations. The iron hexacyanidecan be ferricyanide, ferrocyanide, or a mixture of ferri-/ferrocyanide.These alkali metal ions may include Li⁺, Na⁺, K⁺, or Cs⁺. All of theembodiments and composition and parametric options described above forthe solutions comprising coordination complexes are also considered tobe available and useable independently in solutions and optionsdescribed for solutions comprising iron hexacyanide.

In certain preferred embodiments comprising an iron hexacyanide, thesolutions, preferably stable solutions, are aqueous, comprising twotypes of counterions, those two type of counterions being Na⁺ and K⁺,present in the ranges described above for counterions (i.e., includingthe range of from about 1:10 to about 10:1, about 1:5 to about 5:1,about 1:4 to about 4:1, about 1:3 to about 3:1, about 1:2 to about 2:1,about 1:1.5 to about 1.5:1, or substantially 1:1). An alkaline aqueoussolution, free or substantially free of co-solvents, comprising ironhexacyanide (eitherferricyanide, ferrocyanide, or a mixture offerri-/ferrocyanide) and a substantially equimolar mixture of Na⁺ and K⁺is most preferred.

In further embodiments, the coordination complex, and particularly theiron hexacyanide, is present in a (stable) solution in a concentrationof at least about 0.8 M. In additional independent embodiments, theconcentration of the iron hexacyanide is at least about 0.9 M, about 1M, about 1.2 M, about 1.3 M, about 1.4 M, or at least about 1.5 M, andup to about 3 M, 2.5 M, 2 M, 1.75 M, 1.5 M, or about 1 M. One exemplary,non-limiting embodiment includes a solution wherein the ironhexacyanide, is present in a concentration in a range of from about 1 Mto about 3 M.

The present invention(s) further comprises methods of preparing thesolutions, preferably stable aqueous solutions of iron hexacyanide,having enhanced solubility. Some of these are described in the followingparagraphs. In addition to those embodiments specifically described, itshould be appreciated that these teachings may be combined to provideadditional embodiments, each of which is deemed within the scope of thepresent invention. Also, for the sake of clarity, all solutions orcompositions which may result from the below-described methods areconsidered separate embodiments of the present invention. For example,the molar ratios of the iron hexacyanide salts may be in any ratio so asto satisfy the basic and novel characteristic of enhanced solubility,including those discussed above, although intermediate ratios, includingca. 0.9:1 to about 1.1:1 or substantially 1:1 are preferred. Similarly,the solutions having enhanced solubilities of iron hexacyanide may besubstantially neutral or alkaline. A preference for any of these optionsdepends on the intended use of the solution.

In a first strategy, certain embodiments provide methods of preparingaqueous solutions, preferably stable aqueous solutions of iron(II)hexacyanide, [Fe(CN)₆ ⁴⁻], each method comprising dissolving sufficientamounts of Na₄[Fe(CN)₆] and K₄[Fe(CN)₆] in an amount of aqueous solvent(preferably water substantially free of co-solvents), so as to provide aconcentration of Fe(CN)₆ ⁴⁻ in said solution, at a given temperature,that exceeds the concentration of Fe(CN)₆ ⁴⁻ in either a saturatedsolution of Na₄[Fe(CN)₆] or a saturated solution of K₄[Fe(CN)₆], at thesame temperature. This “boosting” of solubility of Fe(CN)₆ ⁴⁻ insolution, by “dissolving sufficient amounts of Na₄[Fe(CN)₆] andK₄[Fe(CN)₆] in an amount of aqueous solvent (preferably watersubstantially free of co-solvents)” may be accomplished by (a) co-mixingsolid salts of Na₄[Fe(CN)₆] and K₄[Fe(CN)₆] with the aqueous solvent;(b) by admixing a solid salt of Na₄[Fe(CN)₆] with a solution (includinga saturated solution) of K₄[Fe(CN)₆]; or (c) by admixing a solid saltK₄[Fe(CN)₆] with a solution (including a saturated solution) ofNa₄[Fe(CN)₆]. While solid salts of Na₄[Fe(CN)₆] and K₄[Fe(CN)₆] includeanhydrous forms and any hydrate or solvate thereof, Na₄Fe(CN)₆.10H₂O andK₄Fe(CN)₆.3H₂O are preferred starting materials with these methods. This“boosting” methodology may also be applied to the methods which follow.

Where an alkaline solution is desired, additional embodiments of thesemethods further include adding a sufficient amount of an appropriatebase (i.e., so as to maintain the enhanced solubility) to effect andmaintain the desired pH or pH range.

As just described, the enhanced solubility available by this strategy,or the other strategies described herein, may be realized by thepresence of the Na^(+/)K⁺ counterions as provided by the salts of thestarting materials themselves. But since the enhanced solubility appearsto be attributable by the total counterion population in the resultingsolution, such may be also realized by the addition/presence of otherdissolved salts. For example, in related embodiments, (stable) aqueoussolutions of iron(II) hexacyanide, [Fe(CN)₆ ⁴⁻] may be prepared bydissolving sufficient amounts of Na₄[Fe(CN)₆] and/or K₄[Fe(CN)₆] in anamount of solvent comprising Na^(+/)K⁺⁻ containing salts, whichcombination provides the necessary plurality of counterions, in asolution environment, which yields the enhanced solubility. ExemplaryNa^(+/)K⁺ containing salts include, but are not limited to sodium and/orpotassium chloride, carbonate, hydroxide, hydrogen phosphate, nitrate,or sulfate.

In a second strategy, mixtures of basic Na^(+/)K⁺⁻ salts may be reactedwith H₄[Fe(CN)₆], yielding solutions of enhanced [Fe(CN)₆ ⁴⁻]solubility, with the concomitant change in solution pH. For example,other embodiments provide methods of preparing aqueous solutions,preferably stable aqueous solutions of iron(II) hexacyanide (Fe(CN)₆⁴⁻), each method comprising mixing sufficient amounts of H₄[Fe(CN)₆],NaOH, and KOH in sufficient water, so as to provide a concentration ofFe(CN)₆ ⁴⁻ in said solution, at a given temperature, that exceeds theconcentration of Fe(CN)₆ ⁴⁻ in either a saturated solution ofNa₄[Fe(CN)₆] or a saturated solution of K₄[Fe(CN)₆], at the sametemperature. Further embodiments using this strategy include those whereother basic Na^(+/)K⁺⁻ salts—e.g., carbonates or bicarbonate—aresubstituted in whole or in part for the hydroxides.

In a third strategy, sufficient amounts of a calcium salts of iron(II)hexacyanide [Fe(CN)₆ ⁴⁻] are reacted with mixtures of basicNa^(+/)K⁺-salts under conditions which yield solutions of enhanced[Fe(CN)₆ ⁴⁻] solubility, with the concomitant precipitation (andsubsequent removal) of an insoluble calcium salt. For example, one suchseries of embodiments provides methods of preparing aqueous solutions,preferably stable aqueous solutions of iron(II) hexacyanide (Fe(CN)₆⁴⁻), each method comprising: (a) mixing sufficient amounts ofCa₂[Fe(CN)₆], NaOH, and KOH in an amount of water, so as to provide aconcentration of Fe(CN)₆ ⁴⁻ in said solution, at a given temperature,that exceeds the concentration of Fe(CN)₆ ⁴⁻ in either a saturatedsolution of Na₄[Fe(CN)₆] or a saturated solution of K₄[Fe(CN)₆], at thesame temperature; and (b) removing precipitated Ca(OH)₂. Furtherembodiments using this strategy include those where mixed calciumsalts—e.g., CaNa₂[Fe(CN)₆] or CaK₂[Fe(CN)₆]—are substituted in whole orin part for the Ca₂[Fe(CN)₆], where other basic Na^(+/)K⁺⁻ salts—e.g.,carbonates or bicarbonate—are substituted in whole or in part for thehydroxides, and where the precipitated calcium salt (be it calciumcarbonate or hydroxide) is removed.

While other methods may be used, the step of removing precipitatedcalcium salt(s) may be accomplished using, for example, centrifugationand/or (ultra) filtration techniques.

Specific additional embodiments provide solutions, each solutioncomprising (or consisting essentially of) a aqueous alkaline solution,preferably a stable aqueous alkaline solution comprising iron(II)hexacyanide (Fe(CN)₆ ⁴⁻), wherein the concentration of Fe(CN)₆ ⁴⁻ insaid solution, at a given temperature and pH, exceeds the concentrationof Fe(CN)₆ ⁴⁻ in either a saturated aqueous alkaline solution ofNa₄[Fe(CN)₆] or a saturated aqueous alkaline solution of K₄[Fe(CN)₆], atthe same temperature and pH.

It should be appreciated that the descriptions provided above for thepreparation of solutions of iron(II) hexacyanide [Fe(CN)₆ ⁴⁻] alsoprovide analogous methods for preparing solutions of enhanced solubilityof iron(III) hexacyanide [Fe(CN)₆ ³⁻]. This may be accomplished, forexample by substituting the corresponding iron(III) hexacyanide [Fe(CN)₆³⁻] precursor for the iron(II) hexacyanide [Fe(CN)₆ ⁴⁻] described above.In an exemplary, but non-limiting, analogy, certain embodiments providemethods of preparing aqueous solutions, preferably stable aqueoussolutions of iron(III) hexacyanide [Fe(CN)₆ ³⁻], each method comprisingdissolving sufficient amounts of Na₃[Fe(CN)₆] and K₃[Fe(CN)₆] in anamount of water, so as to provide a concentration of Fe(CN)₆ ³⁻ in saidsolution which meets the criteria of enhanced solubility (i.e., aconcentration of Fe(CN)₆ ³⁻ greater than allowed by the solubilitylimits of Na₃[Fe(CN)₆] and K₃[Fe(CN)₆] themselves). These methods mayinclude analogous methods of “boosting” as described above for theiron(II) system, wherein the solid salts of Na₃[Fe(CN)₆] and K₃[Fe(CN)₆]may include anhydrous forms and any hydrate or solvate thereof.

In another approach, solutions of iron(III) hexacyanide [Fe(CN)₆ ³⁻] maybe prepared by oxidizing solutions of iron(II) hexacyanide [Fe(CN)₆ ⁴⁻],either electrochemically or chemically, using appropriate reagents knownin the art. Similarly, solutions of iron(II) hexacyanide [Fe(CN)₆ ⁴⁻]may be prepared by chemically or electrochemically reducing solutions ofiron(III) hexacyanide [Fe(CN)₆ ³]. When prepared in this way, thesolutions resulting from, or compositionally equivalent to, the completeor partial oxidation/reduction of the corresponding precursor (i.e.,mixed iron(II/III) systems) are also considered to be within the scopeof the present invention.

The principles described herein, as well as the solutions derived fromthese principles and comprising a charged metal-ligand coordinationcomplex, generally, and iron hexacyanides, specifically, may be used ina wide variety of applications, including but not limited to energystorage; energy conversion; metal extraction from ores or othermatrices; electro- and electroless plating; providing soluble sources ofcyanide or nitric oxide; electrochemical sensor technology; devicecalibration by electrochemical, spectroscopic, or magnetic means; theproduction of safety paper and other inks, dyestuffs, or dyeformulations; animal feed supplements; electrochromics; and anti-cakingagents. The invention is relevant to processes involving solublecyanometallates and those processes involving non-solution-based uses,such as colloids, thin films, amorphous, or crystalline materials. Theuse of such solutions in each of these applications is considered to bewithin the scope of the present invention.

Further, the specific requirements of each application may require orprefer additional additives or components. For example, solutions usefulfor energy storage and electro-/electroless plating typically requireadditional buffering agents, supporting electrolytes, viscositymodifiers, wetting agents, etc.

Certain embodiments provide solutions useful as electrolytes, eachsolution comprising (or consisting essentially of) any of the (stable)solutions described herein, and further comprising a supportingelectrolyte. The term “supporting electrolyte” is well-known in the artsof electrochemistry and energy storage, and is intended to refer to anyspecies which is redox inactive in the window of electric potential ofinterest and aids in supporting charge and ionic conductivity. In thepresent case, a supporting electrolyte does not substantially compromisethe solubility of the coordination complex (including iron hexacyanidecomplexes). Examples include salts comprising an alkali metal, ammoniumion including an ammonium ion partially or wholly substituted by alkylor aryl groups, halide (e.g., Cl⁻, Br⁻, I⁻), chalcogenide, phosphate,hydrogen phosphate, phosphonate, nitrate, sulfate, nitrite, sulfite,perchlorate, tetrafluoroborate, hexafluorophosphate, or a mixturethereof, and others known in the art. In the case of theferro-/ferricyanide chemistry, sodium potassium hydrogen phosphate is aparticularly useful supporting electrolyte.

In these embodiments, the coordination complex (including the ironhexacyanide complex) may be redox active within the operating window ofelectric potential of interest. In such embodiments, thecoordination/iron hexacyanide complex acts, or is capable of acting, asthe energy storage medium.

The solutions of the present invention, and in particular the enhancedsolubility of the inventive iron hexacyanide complex solutions allowsfor substantially higher theoretical charge/discharge densities forenergy storage devices, including flow batteries, than previouslyavailable with iron hexacyanide. The present invention enables theproduction and use of solutions of high concentrations of metal cyanidecomplexes that are stable in pH ranges unavailable to those making useof the higher solubility of an alkaline earth metal salt relative to thesolubilities of alkali metal salts. The latter counterions enable stablealkaline solutions, but they do not enable concentrations greater thanabout 0.8 M using the methods known in the art, and reports of 0.5-0.6 Mat ambient temperatures are the norm. This is limiting in applicationswhere higher concentrations may be beneficial. These applicationsinclude energy storage, where a 0.8 M solution of [Fe(CN)₆]⁴⁻ representsa theoretical charge/discharge density of only 21.4 Ah/L, but the 1.5 Msolution described herein enables 40.2 Ah/L. The term “theoreticalcharge density” is used to describe the maximum charge/discharge densityfor a given system, based on the relationship that a 1 M solution of a 1electron active species provides 26.8 A-h/L and scales directly withconcentration. Such a description avoids any consideration of deviceinefficiencies deriving from other means, as would be reflected in anexperimentally derived “practical” charge density consideration.Accordingly, certain independent embodiments of the present inventionprovide a (stable) solution or electrolyte comprising iron(II)hexacyanide, iron(III) hexacyanide, or a mixture of iron(II) hexacyanideand iron(III) hexacyanide capable of exhibiting a theoreticalcharge/discharge density of at least about 20 A-h/L, at least about 25A-h/L, at least about 30 A-h/L, at least about 35 A-h/L, at least about40 A-h/L, at least about 45 A-h/L, or at least about 50 A-h/L. Using thescaling factor just described, this corresponds to iron hexacyanideconcentrations on the order of at least 0.8 M, 1 M, 1.1 M, 1.2 M, 1.3 M,1.4 M, 1.5 M, 1.75 M, or about 2 M.

The enhanced solubilities of the coordination/iron hexacyanide complexas described herein may provide solutions which are useful simply forthe high ionic conductivity made available by the present invention,independent of any redox character that the complex may or may notexhibit or be capable of exhibiting. For example, ferrocyanide, byitself, is quite conductive, especially at the high concentrations nowmade available by the present invention. For example, at pH 11, theconcentrated aqueous sodium/potassium solutions of [Fe(CN)₆] (e.g., >1M) are at least as conductive as the corresponding sodium sulfatesolutions. As a result, the user may wish to choose conditions in whichthe operating window of the electric potential of interest is outside ofthe electric potential of the coordination/iron hexacyanide complexcouple. In such cases, it may be useful to add a species which is redoxactive within the operating window of interest.

Accordingly, in some embodiments, the electrolyte may comprise any oneof the solutions already discussed, and further comprise an additionalredox active species. The additional redox active “species” may or maynot comprise a transition metal (e.g., may be entirely organic). Use ofthe term “additional” redox active species acknowledges the fact thatthe coordination complex within the (stable) solutions may or may notalready be redox active, or have a redox potential that is outside theoperating window of electric potential to which the electrolyte is to beexposed. That is, these embodiments describe solutions which mayactually comprise two redox active couples (the coordinationcomplex/iron hexacyanide complex being the first and the “additional”redox active species being the second) each operable within the range offrom about −0.8 V to about 1.8 V, vs. RHE, but the “additional” speciesoperable at a different potential than the coordination complex/ironhexacyanide complex, such that, under certain operating conditions, theenhanced solubility coordination/iron hexacyanide complex may act simplyas the supporting electrolyte, while the “additional” redox activespecies is acting as part of a redox active couple.

The present invention also contemplates those devices, systems, andapplications which take advantage of the enhanced solubilities providedby these inventive solutions. For example, independent embodiments ofthe present invention include those electrochemical cells or half-cells,including those associated with flow batteries, which use or incorporateone of the solutions described herein. For example, solutions based onthe ferro-/ferricyanide couple are known to be useful used in zincferro-/ferricyanide systems, but their historic utility has been limitedby the limited solubilities of sodium ferro-/ferricyanide. See, e.g.,Hollandsworth, R. P., et al., “Zinc/ferrocyanide Battery DevelopmentPhase IV” Lockheed Missiles and Space Company, Inc., contractor report,Sandia Contract DE-AC04-76DP00789, 1985; and US Application PublicationNo. 2011/0244277 (to Gordon, et al.), each of which is incorporated byreference in its entirety for all purposes. These two referencesdescribe a series of technologies and strategies intended to improve theperformance of such zinc ferro-/ferricyanide systems, and each of thesetechnologies and strategies would benefit when incorporating the highconcentrations of the ferro-/ferricyanides solutions available by thepresent invention. Any design, apparatus, and/or operating conditionavailable by combining the teachings of the Hollandsworth and Gordonreferences with the present disclosure is within the scope and areconsidered embodiments of the present invention.

The electrochemical cells of the present invention, including thoseelectrochemical cells which operate as a flow battery cell, and whichtake advantage of the enhanced solubility of the coordination complexes,generally, and the ferro-ferricyanide complexes, specifically, may alsobe configured into larger systems, for example using a cell stackarrangement. Such systems, which include at least oneelectrochemical/flow battery cell as described herein, are consideredadditional embodiments of the present invention.

Also considered within the scope of the present invention are thosemethods useful for operating such an electrochemical/flow battery cellor energy storage system. For example, various embodiments providemethods of operating an electrochemical/flow battery cell or an energystorage system which comprise a solution described herein, each methodcomprising passing a current through said solution so as to effect achange in the oxidation state of the coordination complex. Additionalembodiments provide methods of operating an electrochemical/flow batterycell or an energy storage system comprising a solution described herein,each method comprising passing a current through said solution so as toeffect a change in the oxidation state of an iron hexacyanide complex.

EXAMPLES Example 1 Preparation of Sample

In an exemplary embodiment, solid Na₄Fe(CN)₆.10H₂O (33.89 g, 0.070 mol)and K₄Fe(CN)₆.3H₂O (29.57 g, 0.070 mol) were stirred in 80 mL deionizedwater. To dissolve the solids, sufficient water was then slowly added toprovide a sample containing ca. 1.5 M of Fe(CN)₆ ⁴⁻. This solubility wasunexpected given that the solubilities of Na₄Fe(CN)₆.10H₂O andK₄Fe(CN)₆.3H₂O are each known in the art to be less than 0.7 M at thesame ambient temperatures. The concentration of dissolved Fe(CN)₆ ⁴⁻ ionin this sample was confirmed by UV/visible light spectroscopy asfollows. An aliquot of the 1.5 M Fe(CN)₆ ⁴⁻ solution (20 μL) was addedto 2.0 mL of deionized water, resulting in a dilution factor of 101. Theresulting solution was similarly diluted by a factor of 101 with waterto provide a solution having a Fe(CN)₆ ⁴⁻ concentration of ca. 1.5×10⁻⁴M(total dilution 1/101×1/101˜1/10200) which was analyzed by UV/visiblelight spectroscopy using a 1 cm path-length quartz cuvette. FIG. 1 showsa reference spectrum, plotted by molar extinction coefficient for theferrocyanide anion, which was prepared by dissolving a sufficientquantity of Na₄Fe(CN)₆.10H₂O to give 100.0 mL of a 0.10 M solution anddiluting appropriately for spectrophotometric analysis. FIG. 2A showsthe spectrum obtained for 10200-fold dilution (i.e., two seriesdilutions of a factor of 1:101) of the concentrated Fe(CN)₆ ⁴⁻ solution.The spectrum of the diluted sample yielded an absorbance of 0.046 AU at320 nm, corresponding to a molar extinction coefficient at thiswavelength of ca. 315 M⁻¹ cm⁻¹, and in good agreement with the valuederived from the reference spectrum and literature values. (See, e.g.,Cohen, S. R., Plane, R. A., J. Phys. Chem., 1957, 61, 1096-1100). FIG.2B shows a spectrum obtained using only a 668-fold dilution, yielding amolar extinction coefficient which is within experimental error ofequivalent.

The concentrated mixed cation Fe(CN)₆ ⁴⁻ exhibited good solutionstability, showing no signs of precipitated solid or color change afterstanding at ambient room temperature (ca. 20° C. to about 25° C.) of 4weeks or in refrigerated conditions (ca. 0 to 4° C.) for one week.

Example 2 Cyclic Voltammetry

The 1.5 M [Fe(CN)₆]⁴⁻ solution described in Example 1 was interrogatedby cyclic voltammetry, using a glassy carbon working electrode. FIG. 3.In these experiments, sufficient solid sodium potassium hydrogenphosphate, NaOH, and KOH was added to the 1.5 M [Fe(CN)₆]⁴⁻ solution toyield a working solution having a pH of 11.1 (ratio N^(+/)K⁺˜1) andcontaining 1.5 M [Fe(CN)₆]⁴⁻ and 0.1 M phosphate. These results areconsistent with cyclic voltammograms obtained for more dilute solutionsof [Fe(CN)₆]⁴⁻. See, e.g., Pharr, C. et al., Anal. Chem. 1997, 69,4673-4679 (FIG. 5, showing CV results from 10 mM ferro-/ferricyanidesolution).

In another experiment, Na₄Fe(CN)₆.10H₂O (3.39 g, 0.0070 mol) andK₄Fe(CN)₆. 3H₂O (2.96 g, 0.0070 mol) were stirred in 10 mL water thatwas 1 M in NaOH and 1 M in KOH. After several hours of stirring, thesmall amount of solid remaining is removed by filtration. When analyzedby UV/Visible light spectroscopy in a procedure analogous to thatoutlined above, the total [Fe(CN)₆ ⁴⁻] was determined to be 1.0 M,significantly higher than the ˜0.6 M concentrations reported uponsaturating 2 M NaOH with Na₄Fe(CN)₆.10H₂O. The electrochemical activitywas retained in highly conductive, highly alkaline solutions, as shownin FIG. 4.

Example 3 Energy Storage Capacity

The energy storage capacities of solutions having enhancedconcentrations of the iron hexacyanide were studied by standard methods,using an electrochemical cell having a 5 cm² active area. The resultsshown in FIG. 5 compare the storage capacity of a positive electrolyteat pH of 11 comprising a 0.43 M solution of Na₄Fe(CN)₆ with those of a1.41 M solution prepared by dissolving Na₄Fe(CN)₆ and K₄Fe(CN)₆ when anappropriate quantity of a negative electrolyte is used in both cases sothat the latter does not limit the capacity of the system. The 0.43 Mferrocyanide solution yielded a theoretical charge/discharge density of11.5 Ah/L, and the 1.41 M solution yielded a value of 37.8 Ah/L. Asolution of 1.5 M ferrocyanide yields a theoretical charge/dischargedensity of 40.2 Ah/L.

As those skilled in the art will appreciate, numerous modifications andvariations of the present invention are possible in light of theseteachings, and all such are contemplated hereby. For example, inaddition to the embodiments described herein, the present inventioncontemplates and claims those inventions resulting from the combinationof features of the invention cited herein and those of the cited priorart references which complement the features of the present invention.Similarly, it will be appreciated that any described material, feature,or article may be used in combination with any other material, feature,or article, and such combinations are considered within the scope ofthis invention. The disclosures of each patent, patent application, andpublication cited or described in this document are hereby incorporatedherein by reference, each in its entirety, for all purposes.

What is claimed is the following:
 1. A method comprising: dissolvingH₄[Fe(CN)₆], NaOH and KOH in water, to produce an aqueous solution, soas to provide a concentration of Fe(CN)₆ ⁴⁻ in the aqueous solution thatexceeds the concentration of Fe(CN)₆ ⁴⁻ in either a saturated aqueoussolution of Na₄[Fe(CN)₆] or a saturated aqueous solution of K₄[Fe(CN)₆]in water at the same temperature, wherein the aqueous solution isinterconvertible between a ferricyanide state and a ferrocyanide statewithout forming a precipitate in either state.
 2. The method of claim 1,wherein the aqueous solution is substantially free of a co-solvent. 3.The method of claim 1, wherein the Fe(CN)₆ ⁴⁻ is present in the aqueoussolution at a concentration ranging between about 1 M and about 3 M. 4.The method of claim 1, wherein a molar ratio of sodium ions to potassiumions in the aqueous solution ranges between about 1:10 and about 10:1.5. The method of claim 4, wherein the molar ratio of sodium ions topotassium ions ranges between about 0.9:1 and about 1.1:1.
 6. The methodof claim 1, further comprising adding one or more selected from thegroup consisting of a buffering agent, a supporting electrolyte, aviscosity modifier, and a wetting agent to the aqueous solution.
 7. Amethod comprising: dissolving Ca₂[Fe(CN)₆], NaOH and KOH in water, toproduce an aqueous solution, so as to provide a concentration of Fe(CN)₆⁴⁻ in the aqueous solution that exceeds the concentration of Fe(CN)₆ ⁴⁻in either a saturated aqueous solution of Na₄[Fe(CN)₆] or a saturatedaqueous solution of K₄[Fe(CN)₆] in water at the same temperature,wherein the aqueous solution is interconvertible between a ferricyanidestate and a ferrocyanide state without forming a precipitate in eitherstate.
 8. The method of claim 7, further comprising: removingprecipitated Ca(OH)₂ from the aqueous solution.
 9. The method of claim7, wherein the aqueous solution is substantially free of a co-solvent.10. The method of claim 7, wherein the Fe(CN)₆ ⁴⁻ is present in theaqueous solution at a concentration ranging between about 1 M and about3 M.
 11. The method of claim 7, wherein a molar ratio of sodium ions topotassium ions in the aqueous solution ranges between about 1:10 andabout 10:1.
 12. The method of claim 11, wherein the molar ratio ofsodium ions to potassium ions ranges between about 0.9:1 and about1.1:1.
 13. The method of claim 7, further comprising adding one or moreselected from the group consisting of a buffering agent, a supportingelectrolyte, a viscosity modifier, and a wetting agent to the aqueoussolution.
 14. A method comprising: dissolving CaNa₂[Fe(CN)₆] orCaK₂[Fe(CN)₆], NaOH and KOH in water, to produce an aqueous solution, soas to provide a concentration of Fe(CN)₆ ⁴⁻ in the aqueous solution thatexceeds the concentration of Fe(CN)₆ ⁴⁻ in either a saturated aqueoussolution of Na₄[Fe(CN)₆] or a saturated aqueous solution of K₄[Fe(CN)₆]in water at the same temperature, wherein the aqueous solution isinterconvertible between a ferricyanide state and a ferrocyanide statewithout forming a precipitate in either state.
 15. The method of claim14, further comprising: removing precipitated Ca(OH)₂ from the aqueoussolution.
 16. The method of claim 14, wherein the aqueous solution issubstantially free of a co-solvent.
 17. The method of claim 14, whereinthe Fe(CN)₆ ⁴⁻ is present in the aqueous solution at a concentrationranging between about 1 M and about 3 M.
 18. The method of claim 14,wherein a molar ratio of sodium ions to potassium ions in the aqueoussolution ranges between about 1:10 and about 10:1.
 19. The method ofclaim 18, wherein the molar ratio of sodium ions to potassium ionsranges between about 0.9:1 and about 1.1:1.
 20. The method of claim 14,further comprising adding one or more selected from the group consistingof a buffering agent, a supporting electrolyte, a viscosity modifier,and a wetting agent to the aqueous solution.